In recent years, with increasing volumes of information, advances of optical transmission systems have progressed, enabling larger capacities and longer transmission distances (see, e.g., Japanese Laid-Open Patent Publication No. 2006-245343). With increases in the scale of the optical transmission systems, the point of focus has become the maintenance and improvement of transmission performance, prompting applications of various techniques, such as adoption of a low-noise repeater amplifier and error correction at a transceiving apparatus. For better transmission performance (transmission quality), improving Optical Signal Noise Ratio (OSNR) after transmission and suppressing optical waveform deterioration during transmission have become points of interest. Optical waveform deterioration during transmission is caused by, for example, an interaction between wavelength dispersion occurring in a transmission fiber and nonlinear phenomena such as self phase modulation, cross phase modulation, and four wave mixing.
With the conventional art, although a higher input level (transmission level) of an optical signal to a transmission fiber at the transmission-side leads to an improvement in the OSNR, a deterioration in waveform occurs as well, making it difficult to determine the input level that improves transmission performance. The input level that improves transmission performance varies depending on the type of a transmission fiber, etc. This poses a problem of difficulty in improving transmission performance.